1. Field of the Invention
The present invention relates to a cleaning device including a cleaning blade removing a toner on an image carrier and a magnetic brush roller removing an adhesive substance on the image carrier by a magnetic brush formed of carriers (magnetic particles) and a toner, and also to an image forming apparatus, such as a copier or a printer, provided therewith.
2. Description of Related Art
In an image forming apparatus such as a copier or a printer, a photosensitive drum whose surface is uniformly charged by a charging device is subjected to exposure scanning by an optical scanner, and an electrostatic latent image is formed on the surface thereof in accordance with image information. Then the electrostatic latent image is developed by a developing device by using a toner as a developer to be visualized as a toner image. This toner image is transferred onto paper from the photosensitive drum by a transfer device and then heated and pressurized to be fixed onto the paper by a fixing device, and the paper with the toner image fixed thereon is discharged out of the apparatus, thereby ending a series of image formation operation. The toner remaining on the photosensitive drum without being transferred onto the paper in the transfer process (post-transfer remaining toner) is removed by a cleaning device for collection.
In a process of discharging by the charging device in the image forming apparatus, an ion product such as NOx, SOx, or the like is generated, and this ion product adheres to the surface of the photosensitive drum. Moreover, in an image forming apparatus that applies configuration such that recording paper is conveyed while in contact with the photosensitive drum, an additive agent, paper dust, etc. included in the recording paper adhere to the surface of the photosensitive drum.
Upon the adhesion of the additive agent, the paper dust, etc. to the surface of the photosensitive drum, these adhesive substances absorb moisture to reduce surface potential of the photosensitive drum, and thus charges generated on the surface of the photosensitive drum by the charging device easily move along the surface of the photosensitive drum, causing a phenomenon of image deletion, which deteriorates image quality.
Thus, provided in the cleaning device is a polishing roller formed of an elastic body such as foamed synthetic rubber as means adapted to remove the ion product, the additive agent, the paper dust, etc. adhering to the surface of the photosensitive drum. Rotating this polishing roller while bringing it into contact with the photosensitive drum polishes the surface of the photosensitive drum, and shaving off the adhesive substances cleans the surface of the photosensitive drum, thereby preventing occurrence of the image deletion, etc. as a result of the absorption of the adhesive substances.
However, use of the polishing roller as means adapted to remove the adhesive substances may excessively polish the surface of the photosensitive drum by the polishing roller, causing a problem that life of the photosensitive drum shortens in such a case. In recent circumstance where there are demands for apparatus development that achieves longer life and higher productivity, demands for maintaining stable polishing capability throughout the life have been placed on the photosensitive drum. However, it is difficult to maintain the stable polishing capability due to bending or deformation of the polishing roller.
Moreover, carried out as another countermeasure has been a method of disposing a drum heater near the photosensitive drum and vaporizing moisture absorbed by the adhesive substances through heating by this drum heater to prevent the occurrence of the image deterioration caused by the absorption of the adhesive substances. However, there is a problem that the drum heater is expensive and has high power consumption.
Suggested to solve the problem described above is configuration such that a two-component developer composed of a toner and carriers (magnetic particles) is used as a developer and a rotatable magnetic brush roller is provided in the cleaning device to remove the adhesive substances by polishing the surface of the photosensitive drum by the magnetic brush formed on a surface of the magnetic brush roller. With this configuration, the magnetic brush roller itself does not make contact with the photosensitive drum, and a gap is formed therebetween, and thus the stable polishing capability is maintained without causing uneven polishing regardless of, for example, the bending or deformation of the magnetic brush roller.
However, in the method described above, a small diameter of the photosensitive drum results in a small nip width for the polishing by the magnetic brush, thus causing a problem that a force of polishing the photosensitive drum weakens.
In the method described above, with configuration such that bias with DC bias superposed on AC bias is applied to the magnetic brush roller, reciprocating movement of the developer occurs in a gap between the surface of the photosensitive drum and a surface of a sleeve of the magnetic brush roller, a force in which the developer beats the surface of the photosensitive drum by an electric field increases, and the carriers rub the surface of the photosensitive drum, thus improving capability of removing an impure substance adhering to the surface of the photosensitive drum.
However, in a case where only the carriers as the magnetic brush rub the surface of the photosensitive drum, effect of its rubbing is low. On the contrary, in a state in which the toner adhere to the carriers, the magnetic brush is formed densely on the magnetic brush roller with the toner lying between the carriers, thus improving a force of the magnetic brush pressure-contacting the surface of the photosensitive drum and improving the effect of rubbing the surface of the photosensitive drum by the magnetic brush.
Moreover, in a case where the bias is applied to the magnetic brush roller, toner movement to the photosensitive drum occurs, thus reducing an amount of toner on the magnetic brush formed on the magnetic brush roller and deteriorating the effect of rubbing the surface of the photosensitive drum by the magnetic brush. Thus, the toner needs to be fed to a rubbed portion of the photosensitive drum as needed to stabilize toner-to-carrier ratio (toner weight/carrier weight: hereinafter referred to as T/C) on the magnetic brush. High T/C on the magnetic brush roller results in a large amount of toner held by the magnetic brush, degrading mobility of the magnetic brush, which therefore improves the capability of rubbing the photosensitive drum. However, an excessive increase in the T/C further degrades the mobility of the magnetic brush, thus leading to an increase in the driving torque of the photosensitive drum, which may cause image failure such as a jitter image attributable to ununiform rotation of the photosensitive drum.
On the contrary, low T/C on the magnetic brush roller results in an improvement in the mobility of the magnetic brush, thus not causing the increase in the driving torque of the photosensitive drum as described above. However, the capability of rubbing the surface of the photosensitive drum by the magnetic brush deteriorates, resulting in failure to effectively remove the adhesive substances from the surface of the photosensitive drum.
Thus, the T/C of the magnetic brush formed on the magnetic brush roller needs to be appropriately controlled. However, an amount of the post-transfer toner remaining on the surface of the photosensitive drum after the transfer varies depending on environment, toner chargeability, etc., thus actually making it difficult to control the T/C of the magnetic brush taking the amount of the post-transfer toner into consideration.